1. Field of the Invention
The present invention relates to a device for judging whether or not misfire occurs by detecting an ion current generated between electrodes of spark plug, in particular to a misfire detecting device for internal combustion engines, in which erroneous judgment of misfire is prevented by judging whether or not an ion current detecting means fails.
2. Description of Background Art
In internal combustion engines including a plurality of cylinders, e.g. for cylinders, driven in synchronism with a crank shaft, a microcomputer including an ECU ordinarily controls the engine to repeat four cycles of intake, compression, explosion, and exhaust of fuel, i.e. air-fuel mixture. If fuel compressed by a piston does not optimally and completely burned, an abnormal load is applied to the other cylinders, whereby there are dangers that the engine is destroyed and various troubles are brought about by an outward flow of the unburnt gas.
For example, in order to prevent a damage to a catalyst for treating exhaust gas by the unburnt gas, fuel supply to the cylinder, in which the misfire is detected, is stopped. Accordingly, in order to secure the internal combustion engine and the catalyst, it is necessary to continuously detect whether or not fuel is completely burnt in each of the cylinders. In a conventional technique, a device detecting an ion current generated between gaps of spark plus in explosion strokes and judging whether or not misfire occurs, for example, judging that misfire occurs when the level of the ion current is a predetermined value or less, is proposed.
FIG. 8 is a circuit diagram of a generally-used conventional misfire detecting device of internal combustion engine. In the figure, only a cylinder is shown. However, the device is respectively provided in correspondence with all cylinders in a practical application. Numerical reference 1 designates a power source connected to a battery; numerical reference 2 designates an ignition coil including a primary winding 2a and a secondary winding 2b, one ends of the windings are connected to the power source 1; numerical reference 3 designates a power transistor inserted between the primary winding 2a and a ground; and numerical reference 4 designates a diode for preventing a back current flow, the cathode of the diode is connected to the secondary winding 2b. 
Numerical reference 5 designates a spark plug, one end of which is connected to the secondary winding 2b through the diode 4 and the other end is grounded, wherein the spark plugs are provided in correspondence with a plurality of cylinders so as to be exposed in each combustion chamber. Numerical reference 6 designates a power source for detecting an ion current, the power source is connected to the anode of the diode 4; numerical reference 7 designates a diode for preventing a back current flow, the diode is inserted between the contact of the diode 4 with the spark plug 5 and the power source 6; numerical reference 8 designates a resistor inserted between the power source 6 and a ground; and numerical reference 9 designates an output terminal for detecting the ion current, the output terminal is located in the contact of the power source 6 with the resistor 8.
Next, an operation will be described in reference of FIGS. 8 and 9. In an ignition cycle, the power transistor 3 is controlled to turn on or off by a control signal C from an ECU (not shown). When a primary current I1 flowing through the primary winding 2a is cut off, a secondary voltage V2 having a negative high voltage is induced in the secondary winding 2b. Accordingly, spark is generated in the spark plug 5 and fuel in the combustion chamber is exploded. A discharge period is ordinarily 1 to 1.5 mmsec.
When the fuel is normally exploded in the explosion (ignition) stroke, a large number of positive ions are produced in the combustion chamber. The positive ion flow as the ion current I from the electrode of the spark plug 5 through the diode 7 to the power source 6, and further to the ground through the resistor 8. Accordingly, by detecting the amount of voltage drop in the resistor 8, it is possible to know the level of the ion current I, whereby it is possible to judge whether or not the combustion is normally completed.
The level of ion current I is outputted from the output terminal 9 to the ECU. The ECU judges whether or not the combustion is normally completed in the cylinder which is controlled to ignite. When an error such as misfire is judged to exist, an ignition timing is controlled to feed back, or processes such as fuel cut and cessation of cylinder are conducted to prevent danger.
However, since in the conventional misfire detecting device for internal combustion engine, misfire is judged based on only the ion current I, there is a problem that the ECU erroneously judges that misfire exists in the engine because when wires between the cylinders and the ion current detecting portion or a transmission wire between the output terminal for detecting the ion current and the ECU, and so on are disconnected or shorted or a circuit element is broken it becomes difficult to detect the ion current I.
It is an object of the present invention to solve the above-mentioned problems inherent in the conventional technique and to provide a misfire detecting device for an internal combustion engine comprising: an ion current detecting means for detecting ion currents generated between electrodes of spark plugs corresponding to a plurality of cylinders; an ion current judging means for judging whether or not the ion current exists at each ignition cycle of each cylinder based on the ion current signal from the ion current detecting means; a misfire judging means for judging that a misfire occurs when no ion current is judged to exist based on an ion current judgment signal from the ion current judging means; an ignition coil for generating an ignition signal for each of the spark plugs; an ignition coil signal judging means for judging whether or not the ignition coil signal generated in synchronism with ignition timing of the ignition coil exists; and a failure judging means for judging whether or not an failure of the ion current detecting means occurs based on a result from the ignition coil signal judging means in a case where the ion current signal is not detected.
According to a second aspect of the present invention, there is provided the misfire detecting device for internal combustion engine, wherein the failure judging means judges whether or not a failure of the ion current detecting means exists in a predetermined running range where the ion current signal is detectable.